Distribution of N, Rubisco and photosynthesis in Pinus pinaster and acclimation to light

The light-nitrogen hypothesis suggests canopy photosynthesis is maximized when there is a positive relationship between irradiance received by foliage, its nitrogen content (per unit area N-area), and maximum rate of photosynthesis (A(max)). Relationships among relative irradiance and N-area allocation of nitrogen within the photosynthetic apparatus to Rubisco and chlorophyll, and A(max) were examined in Pinus pinaster Ait. needles up to 6 years of age. Measurements were made before bud break in August 1998, and in May 1999 after the first 'winter' rains. In August, N-area in P. pinaster needles decreased from 5.1 to 5.7 g m(-2) in sunlit 1-year-old needles to 2.3 g m(-2) in shaded 6-year-old needles. In May, N-area was 5-40% less but spatial trends were the same. At both sampling dates, A(max) was less in old shaded needles compared with young sunlit needles, and was thus consistent with the light-nitrogen hypothesis. Relationships between N-area and A(max) were positive at both dates yet varied in strength and form. Allocation of nitrogen within the photosynthetic apparatus was qualitatively consistent with acclimation to light (i.e. Rubisco/Chl decreased with shading), but quantitatively suboptimal with respect to photosynthesis owing to consistent over-investment in Rubisco. This over-investment increased with height in the canopy and was greater in May than in August.